all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/stunserver/stunserver_test.go

@@ -60,8 +60,7 @@ func TestSTUNServer(t *testing.T) {
 
 func BenchmarkServerSTUN(b *testing.B) {
 	b.ReportAllocs()
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := b.Context()
 
 	s := New(ctx)
 	s.Listen("localhost:0")